A. Field of Invention
The invention is directed to the art of planar mold designs for curing long power transmission belts and the like. More particularly, the invention is directed to a mold design incorporating colder and hotter zones separated by transitional zones. The path of heat transfer fluid flow in each zone maximizes desirable heating/cooling effects.
B. Description of the Related Art
Long power transmission belts are often molded and vulcanized in sections using planar molds. The process of molding and vulcanizing is also called curing the belt. The mold surface contains cavities having the desired profile of the belt. In synchronous power transmission belts the mold includes accurately machined cavities for forming the belt teeth. The mold is applied to overlapping or abutting sections of the belt to form the required number of teeth.
In one method for forming the belts, the teeth are formed during curing, by a process called “transfer molding.” During cure, the tooth material is transferred by flowing from the side of the tensile cord opposite the mold cavities to the other side. This transfer fills the cavities of the mold with fabric and elastomer prior to any significant cross-linking in the elastomer, which would prevent further movement or flow. Sections of the belt that have not been molded to the proper shape should be prevented from getting hot enough to vulcanize. When at least some of the belt materials are thermoplastic at the cure temperature, the belt should be cooled before it is removed from the mold. It is economically desirable to fully vulcanize as much of the belt as possible during each mold cycle and to make each cycle time as short as possible.
Temperature variations in the mold can require longer curing cycle times so that the coldest portion is fully cured, but longer curing cycle times can result in unacceptable over-cure in the hotter areas.
Temperature variation also causes variable thermal expansion of the mold and belt materials, which can cause variable tooth pitch within one molded section of the belt.
Molds heated by platens can have areas of poor heat conduction due to gaps and contact resistance resulting in hot or cold spots during rapid heating and cooling. Also, molds with cross-drilled fluid passages tend to be hotter at the edge where the fluid enters, especially during transient operation. A mold with cool ends and a hot center is desirable, but the transition between the ends and the center must be able to provide a steep temperature gradient.
Generally, a transient heating process is usually assumed to begin at ambient temperature. When an initial condition is required that is not considered part of a particular heating/cooling cycle, the process of bringing the mold to initial temperature is called pre-heating or pre-cooling.
For molds used at a constant temperature, the temperature of the heating fluid is often controlled to the temperature desired for the mold. If heat loss from the mold can be eliminated, the mold, starting at ambient temperature, will approach the temperature of the fluid, the difference in temperature between the mold and fluid decreasing in time depending on the thermal properties of the system. These include the geometry, mass, Cp (specific heat), and thermal conductivity of the mold, as well as the Cp, viscosity and flow rate of the fluid. Given a long enough preheat time, the mold will be very close to the fluid temperature and the temperature will be even throughout the mold. Since the heat transfer properties vary throughout the mold, some areas will be hotter than others for times less than the preheat time. More often, there are areas of the mold, like the transition area, where heat loss cannot be eliminated and those areas of the mold approach a steady state temperature that is lower than the heating fluid temperature.
For molds used in a heat/cool cycle, it is required that at least one zone of the mold changes from one temperature to at least one other temperature during the cycle. The heat/cool cycle time may be less than the preheat time needed to achieve uniform mold temperature near the fluid temperature throughout the zone. Curing is a non-linear function of the temperature, so those areas of the mold which increase in temperature slower than others can take longer to reach the desired state of cure. This means that maintaining uniform temperature during transition from one temperature to another is important for uniform cure.
The inventive mold design addresses both the problem of longer than desirable cure times and the problem of inconsistent temperature in the mold. The present invention allows shorter cure cycle times and permit higher cure temperature while minimizing the risk of over-cure.